1. Field of the Invention
The present invention relates to a lens system, and more particularly, to a lens system capable of compensating for chromatic aberration and enlarging the field-of-view, and an objective lens system and an optical projection system using the lens system.
2. Description of the Related Art
In general, objective lens systems or optical projection systems used in various optical displays or optoelectronic devices include a lens system. A conventional lens system includes an optical element having a small magnifying power for aperture aberration correction, which is arranged closer to an image plane, a double-convex lens, and a negative optical element with a concave surface, which is arranged closer to the image plane.
Such a conventional lens system is disclosed in U.S. Pat. No. 4,300,817 entitled “Projection Lens”, issued on Nov. 17, 1981.
FIG. 1 illustrates a structure of the lens system disclosed in U.S. Pat. No. 4,300,817. Referring to FIG. 1, the projection lens system includes three groups G1, G2 and G3. The group G1 includes only a single lens L1 having a very weak positive optical power. The single lens L1 has one plano surface S1 and one aspheric surface S2 and corrects aperture-dependent aberrations. A single lens L2 of the group G2 is a double-convex lens with at least one aspheric surface and provides substantially the entire positive power of the projection lens system. A single lens L3 of the group G3 has a concave image-side surface S5 closest to an image plane and having a negative optical power. An object surface S6 of the single lens L3 that is closest to an object is planar, whereas the image-side surface S5 is aspheric. The single lens L3 serves as a field flattener correcting the Petzval curvature of the single lenses L1 and L2. An element L4 is a radiation shield generally used in monochromic CRT projection systems. Reference character P denotes a surface of a CRT.
However, when applying the conventional lens system to an objective lens system, there are considerable limitations on the correction of aberrations occurring due to the diameter of the optical system pupil since an element that is closest to the image plane is the double-convex lens, further limiting the possibility of increasing objective magnifying power.
In addition, the conventional lens system cannot fully compensate for chromatic aberration, thereby lowering the quality of images projected onto a screen.
Furthermore, as the lens in the form of a meniscus is faced concave to the image surface, the field-of-view of the objective becomes significantly narrower, thereby disabling the use of the objective in compact optical devices, due to its curvature.
In the conventional lens system illustrated in FIG. 1, all of the optical elements are made of acryl material to allow an easy formation of an aspheric surface. However, a refractive index of acryl material varies depending on temperature, so that focal points of the optical elements made of acryl material shift depending on temperature, thereby causing a defocusing or hindering a sharp focusing of the overall lens system.
To overcome these problems, in another conventional lens system disclosed in U.S. Pat. No. 4,776,681, entitled “Projection Lens” and issued on Oct. 11, 1988, a corrector lens unit having a meniscus shape was used. This corrector lens unit has an object side surface having a very large radius of curvature to compensate for an aberration, such as coma aberration, in light that goes in a direction that is not parallel to an optical axis of the lens system.
However, the corrector lens unit of the lens system disclosed in U.S. Pat. No. 4,776,681 has a lens having a convex surface of very large radius of curvature close to a screen, so that the field-of-view becomes too narrow to be applied to smaller optical devices.